Amongst the members mentioned above, document FR 2 497 173, to which reference can be made for further details on this topic, discloses a laminated spherical stop for retaining and hinging a blade to the hub of a rotor for a rotary-wing aircraft. This stop comprises a central portion made up of a stack of alternating layers of flexible material, e.g. elastomer material, and rigid strength members in the form of spherical caps. The stack is arranged between an outer radial support secured to the hub, where “outer” is relative to the axis of the rotor, and inner radial support secured to the root portion of the corresponding blade.
More generally, the elastomeric member presents one of the following two structures. In a first embodiment, particularly suitable for the frequency adapters for a rotor of a rotary-wing aircraft, the elastomer layers are plane sheets, each bonded between two strength members in the form of plates. In a second embodiment, each elastomer layer is tubular and is bonded between an internal strength member and an external strength member which are likewise tubular.
In both cases, the elastomer is stressed in shear by the relative movement between two supports each fixed to a respective one of the two members of the rotor, i.e. to two adjacent blades or to a blade and the hub, between which relative lagging movements need to be damped.
When the elastomeric member is integrated in a laminated stop for retaining and hinging a blade on the hub of a rotor, the elastomer layers of the stop are stressed in shear by the lagging movements, and also by the pitch and flapping movements that are transmitted by the blade to the radially-inner support of the stop. In addition, the elastomer layers are also subjected to stress in compression by the centrifugal forces acting on the blade while the rotor is rotating. In operation, the elastomer is thus pressed dynamically at the frequency of rotation of the rotor, in flapping, in lagging, and above all in pitch.
The mechanical stresses exerted on the elastomer layers of the elastomeric member, regardless of the use to which it is put (laminated stop, frequency adapter, laminated spherical joint, etc.), can lead to cracks forming therein, parallel to the rigid strength members, which will lead in the end to the rigid strength members separating or to the elastomer splitting, and it is even possible for the elastomer to split into two portions. This drawback is particularly significant insofar as proper operation of the elastomeric member depends on good bonding between the various layers. Manufacturers of members of this type have consequently been led to set up separation criteria, based on the lengths of cracks that are visible from the outside, in order to determine when the time has come to replace a laminated stop that has cracks.
Measuring the lengths of cracks requires the use of tools, and in particular of a ruler. That method is particularly difficult to implement, given the generally difficult conditions of access to the elastomeric members because of the large number of components in the vicinity of said members, thereby considerably increasing the length of time required for maintenance operations.